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gas-holder or cylinder.   As soon as a glowing chip is ignited at
the front end and the moisture, which at first collects there, has
disappeared,- the gas jets are turned down  and  finally   ex-
tinguished.    The oxygen  is then stopped,   and  the  straight
calcium chloride tube inserted into the open end of the tube.
* Preliminary Operations.—Grind up a little pure oxalic
acid, and carefully weigh out 0-15 to 0*2 gram (not more) in the
boat   Weigh also the calcium chloride tube and potash appa-
ratus without stoppers or other accessories.    The side tube of
the calcium chloride tube, which carries the bulb, is attached
directly to the combustion tube with a rubber cork.    This cork
should be carefully selected, and should exactly fit the com- •
bustion tube.   The bore hole should be small and smooth, and
it is advisable to dust it with graphite or coat it with a film of
vaseline to prevent the rubber from clinging to  the glass, a.
matter of frequent occurrence unless this precaution is taken.
The cork should be kept exclusively for the combustion.   Push
the side tube of the calcium chloride tube through the hole until
it is flush with the opposite surface, and squeeze the cork tightly
into the combustion tube.   Attach the potash apparatus to the
other limb of the calcium chloride tube by a well-fitting piece
of rubber tubing about 3 cm. (iŁ in.) long, and bring the ends
of the glass as closely as  possible  together.     It   should  be
unnecessary to wind wire round the joint if the rubber is of the
right diameter.   A little vaseline may be used here with advan-
tage, but only in the thinnest film.    The potash apparatus
will require to be supported upon a block or stand.    Remove
the copper spiral from the back of the tube.    Introduce the
boat and push it into position against  the asbestos plug by
means of the spiral which is placed behind it.    Replace the
rubber cork connected with the drying apparatus. The apparatus
will present the appearance shown in Fig. 10.
It must now be tested to see that it is air-tight. For this
purpose, close the open end of the potash apparatus with a
tight stopper and turn on the full pressure from either gas-
holder. After the first few bubbles of air have passed through
the bulbs of the potash apparatus no further movement of
bubbles should appear in any part of the apparatus. If it
withstands this test, the combustion may proceed. Release the
pressure by closing the tap of the gas-holder, screwing i»p the